The instant invention is in the field of methods and apparatus for online analysis of liquid process streams in petroleum refineries and more specifically the instant invention is in the field of methods and apparatus for determining the composition of alkylation catalyst comprising a single phase mixture consisting of strong acid such as hydrofluoric acid (HF) or sulfuric acid (H2SO4, or SA), water (H2O), and hydrocarbons (HC) that include acid soluble oil (ASO), isobutane, alkylate, and the like.
The use of multivariable methods to analyze HF alkylation catalyst, and the motivation to do so, is well known (see U.S. Pat. Nos. 5,681,749; 6,096,553; 7,972,863; 8,211,706; 8,334,142 or 8,751,167). For example, chemometrics have been applied to obtain predictions of % HF, % H2O, and % ASO from spectra measured by online near-infrared (NIR) and Raman spectrometers. Also, multilinear regression (MLR) methods have been used to infer the same properties using outputs from a plurality of univariate sensors integrated into a single analyzer system. These technologies have provided three principal benefits to refiners. First, they reduce the frequency with which samples must be manually obtained from the process and analyzed in the refinery laboratory. Second, the frequency of analysis is practically continuous in contrast to the intermittent lab measurements. But as important as are these two benefits in consideration of the objective to control and optimize alkylation unit operation, minimizing operator exposure is a benefit of paramount concern where the alkylation catalyst contains HF. Due to its toxicity, refiners have long sought means for reliable online analysis so as to minimize the need for operators to obtain samples manually and for the subsequent manual analysis in the refinery laboratory. Reliability concerns both the accuracy of the analytical output and the amount of maintenance required to keep the analyzer system operational.
As regards accuracy, both the spectrometric and the multi-sensor approaches share a common inability to compensate for the effects of variable composition of hydrocarbons in the catalyst (HC). Though wishing to not be bound by any particular understanding of alkylation catalyst chemistry, it is believed that HC comprises a continuum of compounds ranging from isobutane to heavy ASO, the latter consisting of pre-polymers whose molecular weights may be greater than 1000. (In industry parlance, ASO is sometimes referred to as polymer.) The HC between those extremes may include light ASO and perhaps even some alkylate. Rather than having a nominally constant composition, the proportions of these HC components can change as a function of feed quality and operating conditions. Such variation can affect two properties that limit the accuracy of prior art approaches for analyzing HF-containing catalyst (HF catalyst). First, the aggregate density of HC has been estimated to vary by more than about ±10% from a nominal value thought to be typically in the range of about 0.78-0.82 kg/L. Second, the hydrogen-to-carbon ratio (H:C) can decrease as the aggregate density of the hydrocarbons increases, the relative variation estimated as being similar to that for density, e.g. about ±10%. The former can have a proportionate impact on measurement accuracy in the case of a multi-sensor analyzer that assumes HC density is nominally constant. Even worse, the effects of the two types of variation in HC compound each other in chemometric-based NIR and Raman methods, which also are sensitive to the amounts and types of chemical functionality in HC compounds.
Concerning operational reliability, NIR-based HF analyzer systems available to refiners have sampling sub-systems that are somewhat complex and unreliable insofar as they employ tubing and numerous fittings that are susceptible to corrosion and eventually to leakage and also contain at least one automated valve that must be replaced at regular intervals due to seal wear caused by repeated open/close cycling. Furthermore, the practice of enclosing the sampling system in a temperature-controlled cabinet demands layers of safety measures to warn against possible leakage of HF and its accumulation to dangerous levels within the enclosure. Consequently, refiners are not wholly satisfied with extant prior art systems for online analysis of acid catalyst despite the ostensible benefits.